Semiconductor integrated circuit

ABSTRACT

A temperature detector sets the level of a temperature detecting signal to a level indicating a high temperature state, detecting that the chip temperature is higher than a first boundary temperature. The temperature detector sets the level of thereof to a level indicating a low temperature state, detecting that the chip temperature is lower than a second boundary temperature. A control circuit changes its operating state according to the level of the temperature detecting signal. This prevents the operating state of the control circuit from frequently switched even when the chip temperature fluctuates around the boundary temperatures, and accordingly reduces current consumption of the control circuit due to the switching operation. Further, the first and second boundary temperatures set a buffer zone, so that the temperature detector does not detect power supply noises as temperature variation. This can prevent malfunction of the temperature detector and the semiconductor integrated circuit.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2003-375276, filed on Nov. 5, 2003 andNo. 2004-187938, filed on Jun. 25, 2004, the entire contents of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor integrated circuithaving a temperature detector.

2. Description of the Related Art

An SRAM has been conventionally used as a work memory of portableequipment such as a cellular phone. However, memory capacity necessaryfor portable equipment has been increasing year after year. Therefore, aDRAM having dynamic memory cells or a pseudo SRAM is adopted as a workmemory in recent years. A DRAM can have larger memory capacity as a workmemory with the same cost since a memory cell thereof is smaller than amemory cell of the SRAM.

Meanwhile, a memory mounted on a cellular phone needs to be low in powerconsumption in order to make the battery life long. A low standbycurrent is especially important in a cellular phone in order to increasethe continuous standby time. The DRAM and the pseudo SRAM requireregular refresh operation of memory cells even while the portableequipment is in nonoperation, and the refresh operation is a factor ofincreasing the standby current. Therefore, various methods are devisedin order to reduce the standby current in the DRAM and the pseudo SRAM.

For example, some technique has been proposed for reducing the standbycurrent, by taking advantage of the characteristics of the dynamicmemory cell that has the data retention time which gets longer as thechip temperature is higher, and setting a long refresh interval when thechip temperature is lower than a certain boundary temperature to reducethe frequency of the refresh operation (disclosed in, for example,Japanese Unexamined Patent Application Nos. Hei 5-266658, Hei 7-73668,and Hei 3-207084).

FIG. 1 shows temperature-dependent data retention time of a dynamicmemory cell. As described above, the lower the chip temperature is, thelonger the data retention time of the dynamic memory cell is. It ispossible to reduce the standby current by changing the refresh intervalaccording to the boundary temperature Tth detected by a temperaturedetector.

FIG. 2 shows a problematic example of a conventional semiconductorintegrated circuit having a temperature detector. When the semiconductorintegrated circuit operates around a boundary temperature Tth, an outputof the temperature detector varies in a short cycle if heat generationdue to the operation of internal circuits and heat release due tooperation termination of the internal circuits are repeated. As aresult, a control circuit connected to an output of the temperaturedetector changes its operating state (low power operation and normaloperation) in response to the output of the temperature detector. Thisswitching operation increases current consumption of the controlcircuit, so that an effect of reducing the standby current is lowered.

FIG. 3 shows another example of a problem of the conventionalsemiconductor integrated circuit having the temperature detector. Whenthe operation and nonoperation of the internal circuits of thesemiconductor integrated circuit are repeated around the boundarytemperature Tth, the temperature detector sometimes malfunctions sinceit detects power supply noises as temperature variation. Therefore, theoutput of the temperature detector varies with a short cycle. At thistime, since, as in FIG. 2, the operating state of the control circuitconnected to the output of the temperature detector is switched overwith a short cycle, current consumption of the control circuitincreases. Further, the operating state of the control circuit shown inFIG. 3 is switched over irrespective of the chip temperature, so thatthe semiconductor integrated circuit malfunctions.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce current consumptionof a semiconductor integrated circuit having a temperature detector.

It is another object of the present invention to prevent malfunction ofa temperature detector due to noises, thereby preventing malfunction ofa semiconductor integrated circuit.

According to one of the aspects of the semiconductor integrated circuitof the present invention, a temperature detector sets the level of atemperature detecting signal to a level indicating a high temperaturestate when detecting that a chip temperature shifts from low to high andis higher than a first boundary temperature. Further, the temperaturedetector sets the level of the temperature detecting signal to a levelindicating a low temperature state when detecting that the chiptemperature shifts from high to low and is lower than a second boundarytemperature that is different from the first boundary temperature. Acontrol circuit changes its own operating state according to the levelof the temperature detecting signal.

In the semiconductor integrated circuit described above, the boundarytemperature based on which the operating state of the control circuit ischanged from a certain state to another state is different from theboundary temperature based on which the operating state of thereof ischanged from another state to a certain state, so that it is able toprevent the operating state of the control circuit from frequentlyswitched over even when the chip temperature fluctuates around theboundary temperature. As a result, current consumption of the controlcircuit due to the switching operation can be reduced. Further, sincethe first boundary temperature and the second boundary temperature set abuffer zone, the temperature detector does not detect power supplynoises or the like generated due to the operation of internal circuitsas temperature variation. As a result, it is possible to preventmalfunction of the temperature detector and of the semiconductorintegrated circuit.

According to another aspect of the semiconductor integrated circuit ofthe present invention, the temperature detector maintains the level ofthe temperature detecting signal while the chip temperature is betweenthe first boundary temperature and the second boundary temperature.

In the semiconductor integrated circuit described above, maintaining thelevel of the temperature detecting signal while the chip temperature isbetween the first boundary temperature and the second boundarytemperature makes it possible to prevent the operating state of thecontrol circuit from frequently switched over, which can reduce currentconsumption due to the switching of the control circuit.

According to another aspect of the semiconductor integrated circuit ofthe present invention, the temperature detector has a temperaturedetecting unit, a first differential amplifier, a second differentialamplifier, and a flipflop. The temperature detecting unit has a resistorand a bipolar transistor that are connected in series between a powersupply line and a ground line, and generates a detection voltagecorresponding to the chip temperature from a connecting node of theresistor and the bipolar transistor. The first differential amplifiercompares a first reference voltage corresponding to the first boundarytemperature with the detection voltage. The second differentialamplifier compares a second reference voltage corresponding to thesecond boundary temperature with the detection voltage. The flipflopgenerates the level of the temperature detecting signal according toresults of the comparisons from the first and second differentialamplifiers.

In the semiconductor integrated circuit described above, it is possibleto monitor the chip temperature as the detection voltage by takingadvantage of temperature-dependent changes in a threshold voltage of thebipolar transistor. The first and second differential amplifiers areused to compare the detection voltage with the first and secondreference voltages corresponding to the first and second boundarytemperatures respectively, which makes it possible to detect the changein the chip temperature accurately with a simple circuit and to surelyswitch over the operating state of the control circuit.

According to another aspect of the semiconductor integrated circuit ofthe present invention, the temperature detector has a temperaturedetecting unit, a basic differential amplifier, a first differentialamplifier, a second differential amplifier, and a flipflop. Thetemperature detecting unit has a resistor and a bipolar transistor thatare connected in series between a power supply line and a ground line,and generates a detection voltage corresponding to the chip temperaturefrom a connecting node of the resistor and the bipolar transistor. Thebasic differential amplifier compares a basic reference voltage and thedetection voltage to output a result of the comparison as a basicdetection voltage. The first differential amplifier compares a firstreference voltage corresponding to the first boundary temperature withthe basic detection voltage. The second differential amplifier comparesa second reference voltage corresponding to the second boundarytemperature with the basic detection voltage. The flipflop generates thelevel of the temperature detecting signal according to results of thecomparisons from the first and second differential amplifiers.

Moreover, the change in the detection voltage (basic detection voltage)inputted to the first and second differential amplifiers is made steepby the basic differential amplifier's amplification of the detectionvoltage outputted from the temperature detecting unit. This enablessecure generation of the temperature detecting signal when the first andsecond boundary temperatures are close to each other, even if thefluctuation or others in the manufacturing conditions of thesemiconductor integrated circuit causes variances in the characteristicsof the first and second differential amplifiers, and the variances causean offset voltage.

According to another aspect of the semiconductor integrated circuit ofthe present invention, a resistor array is constituted of a plurality ofresistors arranged in series between the power supply line and theground line. The resistor array generates the first and second referencevoltages from connecting nodes of predetermined resistors respectively.A variable resistor unit is connected in series to the resistor arrayand a resistance value thereof is changeable. The first and secondreference voltages are adjusted by changing the resistance value of thevariable resistor. Therefore, it is possible to form a circuit in whichthe first and second reference voltages are adjustable without anyincrease in the circuit scale.

According to another aspect of the semiconductor integrated circuit ofthe present invention, a first variable resistor and a second variableresistor are disposed on a power supply line side and a ground line sideof the resistor array, which enables easy and fine adjustment of thefirst and second reference voltages.

According to another aspect of the semiconductor integrated circuit ofthe present invention, a sum of resistance values of the first andsecond variable resistors is always made constant, which makes itpossible to make constant a difference between the first and secondreference voltages to increase or decrease the first and secondreference voltages. As a result, the temperature detector can change thelevel of a temperature detecting signal whenever a certain degree oftemperature change occurs.

According to another aspect of the semiconductor integrated circuit ofthe present invention, the resistance value of the variable resistorunit is preset by a ROM circuit. This makes it possible to manufacture asemiconductor integrated circuit with an optimum characteristic whichgenerates the first and second reference voltages with optimum valuesaccording to a fluctuation in manufacturing conditions or to productspecifications.

According to another aspect of the semiconductor integrated circuit ofthe present invention, a voltage generator generates a plurality ofkinds of voltages. A switch circuit selects two kinds from the pluralityof kinds of voltages to output the selected voltages as a first and asecond reference voltage. A ROM circuit presets the voltages to beselected by the switch circuit.

In the semiconductor integrated circuit described above, plurality ofthe first and second reference voltages can be generated by selectingthe switch circuit according to values set by the ROM circuit. Thismakes it possible to manufacture a semiconductor integrated circuit withan optimum characteristic in conformity with fluctuations in themanufacturing conditions and with the product specifications.

According to another aspect of the semiconductor integrated circuit ofthe present invention, a memory array has dynamic memory cells. Thecontrol circuit is a refresh timer configured to change a generationcycle of a refresh request signal for refreshing the memory cell,according to the level of the temperature detecting signal.

In the semiconductor integrated circuit described above, the refreshcycle of the memory cell is changed according to the chip temperature,which makes it possible to reduce power consumption of the semiconductorintegrated circuit.

According to another aspect of the semiconductor integrated circuit ofthe present invention, a command decoder decodes a read command signaland a write command signal that are access requests supplied via anexternal terminal. An operation control circuit outputs a timing signalfor putting the memory array into operation in order to execute anaccess operation in response to the read command signal and the writecommand signal and a refresh operation in response to the refreshrequest signal. The operation control circuit has an arbiter configuredto determine which one of the access operation and the refresh operationis to be given priority when the read command signal or the writecommand signal conflict with the refresh request signal.

The refresh timer can be operated efficiently to reduce standby currentsince the semiconductor integrated circuit described above has thearbiter configured to determine which of the access operation and therefresh operation is executed first when the read command or the writecommand conflict with the refresh command.

According to another aspect of the semiconductor integrated circuit ofthe present invention, during a normal operation mode, a command decoderdecodes a read command signal and a write command signal that are accessrequests supplied via an external terminal and a self refresh commandsignal for changing the normal operation mode to a self refresh mode. Anoperation control circuit outputs a timing signal for putting the memoryarray into operation in order to execute an access operation in responseto the read command signal and the write command signal and a refreshoperation in response to the refresh request signal. A refresh timerstarts operating when the command decoder decodes the self refreshcommand signal.

In the semiconductor integrated circuit described above, it is possibleto efficiently operate the refresh timer to reduce standby current (selfrefresh current) since the semiconductor integrated circuit has the selfrefresh mode.

According to another aspect of the semiconductor integrated circuit ofthe present invention, after shifting to the self-refresh mode from thenormal operation mode, the refresh timer fixes the generation cycle ofthe refresh request signal to a short cycle irrespective of the level ofthe temperature detecting signal until it generates a predeterminednumber of refresh request signals. After completion of the generation,the refresh timer changes the generation cycle according to the level ofthe temperature detecting signal. The internal circuits of thesemiconductor integrated circuit are sometimes in an unstable stateimmediately after shifting to the self-refresh mode, and the oscillationcycle of a self timer is sometimes unstable. In such cases, fixing thegeneration cycle of the refresh request signal to a short cycle makes itpossible to prevent loss of data retained in memory cells. Therefore, itis possible to improve reliability of the data retained in the memorycells.

According to another aspect of the semiconductor integrated circuit ofthe present invention, after the level change in the temperaturedetecting signal, the refresh timer change the generation cycle of therefresh request signal after it generates a predetermined number ofrefresh request signals. Changing the generation cycle with a time lagsince the level change in the temperature detecting signal can surelyprevent the temperature detector and the semiconductor integratedcircuit from malfunctioning even if the temperature detector frequentlymakes a detection due to power supply noises and the like.

According to another aspect of the semiconductor integrated circuit ofthe present invention, the generation cycle is lengthened not after thelevel change in the temperature detecting signal due to the chiptemperature shift from high to low, but after completion of generationof a predetermined number of refresh request signals according to thelevel change. Further, after the level change in the temperaturedetecting signal due to the chip temperature shift from low to high, thegeneration cycle is shortened in response the level change. When thelevel of the temperature detecting signal changes due to the shift ofthe chip temperature from high to low, that is, reliability of dataretained in memory cells is lowered, delaying the shift timing canprevent deterioration in the reliability of data retained in the memorycells.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a characteristic chart showing temperature-dependent dataretention time of a dynamic memory cell;

FIG. 2 is an explanatory chart showing a problematic example of aconventional semiconductor integrated circuit having a temperaturedetector;

FIG. 3 is an explanatory chart showing another problematic example ofthe conventional semiconductor integrated circuit having the temperaturedetector;

FIG. 4 is a block diagram showing a first embodiment of thesemiconductor integrated circuit of the present invention;

FIG. 5 is a block diagram showing a temperature detector and a refreshtimer shown in FIG. 4 in detail;

FIG. 6 is a waveform chart showing the operation of the temperaturedetector shown in FIG. 5;

FIG. 7 is a waveform chart showing the operation of the temperaturedetector and the refresh timer according to variation in chiptemperature in the first embodiment;

FIG. 8 is a block diagram showing a second embodiment of thesemiconductor integrated circuit of the present invention;

FIG. 9 is a circuit diagram showing a reference voltage setting circuitand a reference voltage generator shown in FIG. 8 in detail;

FIG. 10 is a circuit diagram showing an essential part of a thirdembodiment of the semiconductor integrated circuit of the presentinvention;

FIG. 11 is a waveform chart showing the operation of a temperaturedetector shown in FIG. 10;

FIG. 12 is a block diagram showing a fourth embodiment of thesemiconductor integrated circuit of the present invention;

FIG. 13 is a circuit diagram showing a temperature detector shown inFIG. 12 in detail;

FIG. 14 is a waveform chart showing the operation of the temperaturedetector and a refresh timer according to variation in chip temperaturein the fourth embodiment;

FIG. 15 is a block diagram showing a fifth embodiment of thesemiconductor integrated circuit of the present invention;

FIG. 16 is a block diagram showing a sixth embodiment of thesemiconductor integrated circuit of the present invention;

FIG. 17 is a circuit diagram showing in detail a reference voltagesetting circuit and a reference voltage generator shown in FIG. 16;

FIG. 18 is a circuit diagram showing an example of the reference voltagegenerator shown in FIG. 17;

FIG. 19 is a circuit diagram showing an essential part of a seventhembodiment of the semiconductor integrated circuit of the presentinvention;

FIG. 20 is a block diagram showing an eighth embodiment of thesemiconductor integrated circuit of the present invention;

FIG. 21 is a waveform chart showing the operations of a temperaturedetector and a refresh timer in accordance with a change in chiptemperature in the eighth embodiment;

FIG. 22 is a block diagram showing a ninth embodiment of thesemiconductor integrated circuit of the present invention;

FIG. 23 is a waveform chart showing the operations of a temperaturedetector and a refresh timer in accordance with a change in chiptemperature in the ninth embodiment; and

FIG. 24 is a circuit diagram showing another example of the referencevoltage generator shown in FIG. 17.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be explainedusing the drawings. The double circles in the drawings representexternal terminals. In the drawings, each signal line shown by the heavyline is constituted of a plurality of lines. Part of a block to whichthe heavy line is connected is constituted of a plurality of circuits.The same reference numerals and symbols as those of the externalterminals are used to designate signals supplied via the externalterminals. The same reference numerals and symbols as those of thesignals are used to designate signal lines through which the signals aretransmitted.

FIG. 4 shows a first embodiment of the semiconductor integrated circuitof the present invention. This semiconductor integrated circuit isformed on a silicon substrate as a pseudo SRAM, using a CMOS process.The pseudo SRAM has a memory core of a DRAM and an interface of an SRAM.The pseudo SRAM executes a refresh operation periodically inside a chipwithout receiving an external refresh command and retains data writtento memory cells. The pseudo SRAM is used as a work memory mounted on,for example, a cellular phone. A read operation and a write operationare executed in response to command signals CMD (a read command signaland a write command signal) supplied via an external terminal.

The pseudo SRAM has a command input circuit 10, a reference voltagegenerator 12, a temperature detector 14, a refresh timer 16, a refreshaddress generator 18, an address input circuit 20, a data input/outputcircuit 22, an operation control circuit 24, an address switchingcircuit 26, and a memory core 28. Note that only essential signalsnecessary for explaining the present invention are shown in FIG. 4.

The command input circuit 10 (command decoder) receives the commandsignal CMD (for example, a chip enable signal /CE, a write enable signal/WE, an output enable signal /OE, or the like) supplied via a commandterminal. The command input circuit 10 decodes the received commandsignal CMD (access request) to output an internal command signal ICMDfor putting the memory core 28 into operation.

The reference voltage generator 12 generates a threshold voltage Vth1(first reference voltage) and a threshold voltage Vth2 (second referencevoltage). The threshold voltages Vth1, Vth2 correspond to a firstboundary temperature Tth1 and a second boundary temperature Tth2 whichwill be described later, respectively.

The temperature detector 14 sets a temperature detecting signal TDET tolow level indicating a high temperature state when detecting that thechip temperature of the pseudo SRAM shifts from a low temperature to ahigh temperature to be higher than the boundary temperature Tth1(represented by the threshold voltage Vth1). The temperature detector 14sets the level of the temperature detecting signal TDET to high levelindicating a low temperature state when detecting that the chiptemperature shifts from a high temperature to a low temperature to belower than the boundary temperature Tth2 (represented by the thresholdvoltage Vth2). The temperature detector 14 maintains the level of thetemperature detecting signal TDET when the chip temperature is betweenthe boundary temperatures Tth1, Tth2. The boundary temperature Tth1 ishigher than the boundary temperature Tth2.

The refresh timer 16 generates in a predetermined cycle a refreshrequest signal RREQ for refreshing a memory cell MC. The refresh timer16 sets a timer cycle long when the temperature detecting signal TDET isat high level and sets the timer cycle short when the temperaturedetecting signal TDET is at low level. In other words, the outputfrequency of the refresh request signal RREQ is low when the chiptemperature is low and is high when the chip temperature is high.

The refresh address generator 18 executes a count operation in responseto the refresh request signal RREQ to output a refresh address signalRFA constituted of a plurality of bits. The refresh address signal RFAis a row address signal for selecting a word line WL.

The address input circuit 20 receives an address signal ADD suppliedfrom an address terminal and outputs the received signal as a rowaddress signal RA and a column address signal CA. The row address signalRA is used for selecting the word line WL. The column address signal CAis used for selecting a bit line BLZ (or BLX).

The data input/output circuit 22 outputs read data, which is transferredfrom the memory core 28 via a common data bus CDB, to a data terminal DQ(for example, 16 bits) at the time of the read operation. The datainput/output circuit 22 receives write data supplied via the dataterminal DQ to transfer the received data to the memory core 28 via thecommon data bus CDB at the time of the write operation.

The operation control circuit 24 has an arbiter 25 that determines towhich one of the internal command signal ICMD and the refresh requestsignal RREQ, which are inputted asynchronously, priority should be givenwhen they conflict with each other. The operation control circuit 24outputs a refresh signal REFZ when the refresh operation is executed inresponse to the refresh request signal RREQ. The refresh operation isexecuted by the arbiter 25 between the read operations or the writeoperations executed in response to the read command or the write commandsupplied from an external part of the pseudo SRAM. In other words, therefresh operation is automatically executed inside the pseudo SRAM. Theoperation control circuit 24 outputs a timing signal TIMING fordetermining the operation timing of a plurality of control circuits (aword decoder WDEC, a sense amplifier SA, and so on to be describedlater) in the memory core 28, in response to the internal commandsignals ICMD (the read command signal and the write command signal) orthe refresh request signal RREQ (refresh command signal) to which thearbiter 25 has determined to give priority.

The address switching circuit 26 outputs the row address signal RA as aninternal row address signal IRA while receiving the low level refreshsignal REFZ (during the read operation, the write operation, or astandby period). The address switching circuit 26 outputs the refreshaddress signal RFA as the internal row address signal IRA whilereceiving the high level refresh signal RFEZ (during the refreshoperation). This means that the externally supplied row address signalRA is selected during the read operation, the write operation, and thestandby period, and the internally generated refresh address signal RFAis selected during the refresh operation.

The memory core 28 has the word decoder WDEC, the sense amplifier SA, aprecharging circuit PRE, a memory array ARY, a column decoder CDEC, asense buffer SB, and a write amplifier WA. The operating timings of thecircuits except the memory array ARY are set by the timing signalsTIMING respectively.

The word decoder WDEC selects the word line WL corresponding to theinternal row address signal IRA. The sense amplifier SA amplifies avoltage difference between bit lines BLZ, BLX at the time of the readoperation, the write operation, and the refresh operation. Theprecharging circuit PRE sets the bit lines BLZ, BLX to predeterminedvoltages while the memory core 28 is in nonoperation.

During the read operation and the write operation, the column decoderCDEC selects a column switch for connecting the bit lines BLZ, BLX andthe data bus DB according to the column address signal CA, and turns onthe selected column switch in synchronization with a column line controlsignal CLZ. The sense buffer SB amplifies a signal amount of the readdata on the data bus DB at the time of the read operation to output itto the common data bus CDB. The write amplifier WA amplifies a signalamount of the write data on the common data bus CDB at the time of thewrite operation to output it to the data bus DB.

The memory array ARY has the plural dynamic memory cells MC arranged inmatrix, and the plural word lines WL and the plural bit line pairs BLZ,BLX connected to the memory cells MC. Each of the memory cells MC, whichare the same as typical DRAM memory cells, has a capacitor (memory node)for retaining data as charges and a transfer transistor disposed betweenthis capacitor and the bit line BL. A gate of the transfer transistor isconnected to the word line WL.

FIG. 5 shows the temperature detector 14 and the refresh timer 16 shownin FIG. 4 in detail. The temperature detector 14 has a temperaturedetecting unit 14 a, a first differential amplifier 14 b, a seconddifferential amplifier 14 c, a flipflop 14 d, and so on. The temperaturedetecting unit 14 a has a resistor R1 (for example a diffusionresistance) and a bipolar transistor BP1 that are connected between aninternal power supply line V1 and a ground line VSS. The temperaturedetecting unit 14 a generates a detection voltage corresponding to thechip temperature from a connecting node N01 of the resistor R1 and thebipolar transistor BP1. For example, a threshold voltage of the bipolartransistor BP1 lowers as the chip temperature rises, therefore, voltagedrops at the node N01.

The differential amplifier 14 b compares the threshold voltage Vth1representing the first boundary temperature Tth1 and the detectionvoltage N01. An output node N02 of the differential amplifier 14 bchanges to high level when the detection voltage N01<the thresholdvoltage Vth1, and changes to low level when the detection voltageN01>the threshold voltage Vth1. The differential amplifier 14 c comparesthe threshold voltage Vth2 representing the second boundary temperatureTth2 and the detection voltage N01. An output node N04 of thedifferential amplifier 14 c changes to high level when the detectionvoltage N01>the threshold voltage Vth2, and changes to low level whenthe detection voltage N01<the threshold voltage Vth2. Invertersconnected to outputs of the differential amplifiers 14 b, 14 c,respectively, form and invert waveforms of the nodes N02, N04 and outputthe inverted waveforms to the flipflop 14 d via nodes N03, N05.

The flipflop 14 d changes the temperature detecting signal TDET to lowlevel when the node N03 changes from high level to low level, andchanges the temperature detecting signal TDET to high level when thenode N05 changes from high level to low level.

The refresh timer 16 has a ring oscillator 16 a, a frequency divider 16b, and a selector 16 c. The ring oscillator 16 a has inverters ofodd-number stages connected in cascade and outputs an oscillation signalwith a predetermined cycle. The frequency divider 16 b has ½ frequencydividers of plural stages connected in cascade for dividing thefrequency of the oscillation signal. The selector 16c selects either oneof the frequency divided signals outputted from predetermined two of the½ frequency dividers according to the logic level of the temperaturedetecting signal TDET and outputs the selected frequency divided signalas the refresh request signal RREQ. Note that the ½ frequency dividersoutputting the frequency divided signals to be inputted to the selector16 c are not limited to the two ½ frequency dividers shown in thedrawing but may be determined according to the design and specificationsof the pseudo SRAM.

FIG. 6 shows the operation of the temperature detector 14 shown in FIG.5. The temperature detecting unit 14 a shown in FIG. 5 generates avoltage in the node N01 according to the chip temperature. When the chiptemperature changes from a low temperature to a high temperature toexceed the boundary temperature Tth2 (FIG. 6( a)), the output node N04of the differential amplifier 14 c changes from high level to low level,and the node N05 changes from low level to high level (FIG. 6( b)). Atthis time, since the node N03 is at high level, an output (TDET) of theflipflop 14 d maintains high level (FIG. 6( c)).

When the chip temperature changes from a low temperature to a hightemperature to exceed the boundary temperature Tth1 (FIG. 6( d)), theoutput node N02 of the differential amplifier 14 b changes from lowlevel to high level, and the node N03 changes from high level to lowlevel (FIG. 6( e)). At this time, since the node N05 is at high level,the output (TDET) of the flipflop 14 d changes from high level to lowlevel (FIG. 6( f)).

Conversely, when the chip temperature changes from a high temperature toa low temperature to be lower than the boundary temperature Tth1 (FIG.6( g)), the output node N02 of the differential amplifier 14 b changesfrom high level to low level, and the node N03 changes from low level tohigh level (FIG. 6( h)). At this time, since the node N05 is at highlevel, the output (TDET) of the flipflop 14 d maintains low level (FIG.6( i)).

When the chip temperature changes from a high temperature to a lowtemperature to be lower than the boundary temperature Tth2 (FIG. 6( j)),the output node N04 of the differential amplifier 14 c changes from lowlevel to high level, and the node N05 changes from high level to lowlevel (FIG. 6( k)). At this time, since the node N03 is at high level,the output (TDET) of the flipflop 14 d changes from low level to highlevel (FIG. 6( l). Thus, the temperature detector 14 has a Schmitttrigger function and the temperature detecting signal TDET indicates apreceding value when the chip temperature is between the boundarytemperatures Tth1, Tth2.

FIG. 7 shows the operation of the temperature detector 14 and therefresh timer 16 according to the change in the chip temperature in thefirst embodiment. The temperature detecting signal TDET changes fromhigh level to low level only when the chip temperature exceeds theboundary temperature Tth2 and further exceeds Tth1 (FIG. 7( a)).Further, the temperature detecting signal TDET changes from low level tohigh level only when the chip temperature goes lower than the boundarytemperature Tth1 and further goes lower than Tth2 (FIG. 7( b)). In otherwords, when the chip temperature fluctuates around the boundarytemperature Tth1 (FIG. 7( c)), the boundary temperature Tth2 (FIG. 7( d,e)), and between the boundary temperatures Tth1, Tth2 (FIG. 7( f, g)),the level of the temperature detecting signal TDET does not change. Inthis manner, in the present invention the boundary temperatures Tth1,Tth2 set a buffer zone, so that it is possible to prevent the outputlevel of the temperature detecting signal TDET from frequently changingdue to slight change in the chip temperature or power supply noises thatare caused by the operation of the internal circuits of the pseudo SRAM.Therefore, the operation of the temperature detector 14 can be morestabilized. As a result, it is possible to prevent malfunction of thetemperature detector 14, thereby preventing malfunction of the pseudoSRAM.

When the temperature detecting signal TDET is at high level, the refreshtimer 16 makes the generation interval of the refresh request signalRREQ longer. At a low chip temperature, the data retention time of thememory cell MC gets long, so that data retained in the memory cell MCare not lost even the refresh frequency is lowered. On the other hand,At a high chip temperature, the data retention time of the memory cellMC gets short, so that it is necessary to prevent loss of the dataretained in the memory cell MC by increasing the refresh frequency. Thechange in the refresh frequency according to the chip temperature makesit possible to prevent unnecessary operation of the refresh timer 16 andso on to reduce the standby current. Moreover, it is possible to preventthe refresh cycle from frequently switched when the temperaturefluctuates around the boundary temperature Tth1 or Tth2. Consequently,it is able to prevent increase in current consumption of the temperaturedetector 14 and the refresh timer 16 and increase in the standby currentwhich are caused by the switching operation.

In this embodiment described above, the logic level of the temperaturedetecting signal TDET outputted by the temperature detector 14 ischanged using the two boundary temperatures Tth1, Tth2 as references,and the logic level of the temperature detecting signal TDET ismaintained when the chip temperature is between the boundarytemperatures Tth1, Tth2, which makes it possible to prevent frequentlychanging the cycle of the refresh request signal RREQ even when the chiptemperature fluctuates around the boundary temperature Tth1 or Tth2. Asa result, it is possible to lower the frequency of the switchingoperation in the refresh timer 16 for changing the cycle of the refreshrequest signal RREQ (the refresh cycle of the memory cell MC).Therefore, it is possible to reduce the standby current of the pseudoSRAM having the arbiter 25 that determines the order in which the accessoperation and the refresh operation are executed.

The temperature detecting unit 14 a of the temperature detector 14 iscapable of monitoring the chip temperature as the detection voltage N01by utilizing the temperature dependent change in the threshold voltageof the bipolar transistor BP1. Further, the differential amplifiers 14b, 14 c compare the detection voltage N01 with the threshold voltagesVth1, Vth2 corresponding to the boundary temperatures Tth1, Tth2respectively, so that the change in the chip temperature can be detectedaccurately with a simple circuit.

FIG. 8 shows a second embodiment of the semiconductor integrated circuitof the present invention. The same reference numerals and symbols areused to designate the same elements as the elements explained in thefirst embodiment, and detailed explanation thereof will be omitted. Thesemiconductor integrated circuit of this embodiment is formed on asilicon substrate as a pseudo SRAM, using a CMOS process.

The pseudo SRAM has a reference voltage generator 30 in place of thereference voltage generator 12 of the first embodiment. Further, areference voltage setting circuit 32 is additionally formed. The otherconfiguration is substantially the same as that of the first embodiment.

The reference voltage setting circuit 32 outputs four-bit settingsignals SET in order to initially set respective values of thresholdvoltages Vth1, Vth2 to be generated by the reference voltage generator30. The logic of the setting signals SET is fixed during manufacturingprocesses of the pseudo SRAM. The reference voltage generator 30generates the threshold voltages Vth1, Vth2 having values according tothe logic of the setting signals SET.

FIG. 9 shows the reference voltage setting circuit 32 and the referencevoltage generator 30 shown in FIG. 8 in detail. In this example, onlycircuits for generating the threshold voltage Vth1 are shown. The pseudoSRAM has the same circuits as those in FIG. 9 for generating thethreshold voltage Vth2.

The reference voltage setting circuit 32 has a ROM circuit 32 a thatoutputs fuse signals FS1, FS0 whose logic is fixed during themanufacturing processes of the pseudo SRAM and a decoder 32 b thatdecodes the fuse signals FS1, FS0 and outputs the setting signals SET(SET11, SET10, SET01, SET00). The ROM circuit 32 a has two ROM units 32c, 32 d. Each of the ROM units 32 c, 32 d has a fuse and an nMOStransistor that are connected in series between an internal power supplyline VII and a ground line VSS, and an inverter connected to aconnecting node of the fuse and the nMOS transistor. The nMOS transistoris connected to the internal power supply line VII at a gate thereof tobe constantly on and functions as a high-resistance resistor.

The ROM unit (32 c or 32 d) where the fuse exists outputs the fusesignal (FS1 or FS0) at low level. The ROM unit (32 c or 32 d) where thefuse has blown out outputs the fuse signal (FS1 or FS0) at high level.Each of the two fuses is blown out or not blown out according tomanufacturing specifications in the manufacturing processes of thepseudo SRAM, so that the decoder 32 b sets only one of the settingsignals to low level. Note that the tail numbers of the setting signalsSET11, SET10, SET01, SET00 represent the logic of the fuse signals FS1,FS0. For example, when the logic of the fuse signals FS1, FS0 is “10” inthe binary number, the setting signal SET10 maintains low level and theother setting signals SET11, SET01, SET00 maintain high level.

The reference voltage generator 30 has a plurality of resistorsconnected in series between the internal power supply line VII and theground line VSS and also has a switch circuit 30 a for connecting one ofconnecting nodes of the adjacent two resistors to an output node of thethreshold voltage Vth1. The switch circuit 30 a is constituted of aplurality of sets of a CMOS transmission gate and an inverter, and whenthe setting signal SET is at low level, the corresponding CMOStransmission gate turns on. In this example, four kinds of the thresholdvoltages Vth1 are generated according to the logic of the settingsignals SET (SET11, SET10, SET01, SET00). The resistance values are setaccording to the four generated threshold voltages Vth1 respectively.

This embodiment has described an example where the reference voltagegenerator 30 is formed for each of the threshold voltages Vth1, Vth2.However, the plural resistors connected in series between the internalpower supply line VII and the ground line VSS may be used in common forgenerating the threshold voltages Vth1, Vth2, thereby generating thethreshold voltages Vth1, Vth2 using one reference voltage generator.

The same effects as those in the above-described first embodiment arealso obtainable in this embodiment. Moreover, in this embodiment, thereference voltage setting circuit 32 and the switch circuit 30 a enablethe generation of the plural kinds of the threshold voltages Vth1, Vth2.Consequently, a pseudo SRAM having an optimum characteristic can bemanufactured according to the fluctuation in manufacturing conditions oraccording to product specifications (power consumption specifications).

FIG. 10 shows an essential part of a third embodiment of thesemiconductor integrated circuit of the present invention. The samereference numerals and symbols are used to designate the same elementsas the elements explained in the first embodiment, and detailedexplanation thereof will be omitted. In this embodiment, a temperaturedetector 34 is different from the temperature detector 14 of the firstand second embodiments. The other configuration is substantially thesame as that of the first and second embodiments. Therefore, only thetemperature detector 34 is shown in FIG. 10.

The temperature detector 34 is constituted of the temperature detector14 shown in FIG. 5 plus a basic differential amplifier 34 a. The basicdifferential amplifier 34 a is disposed between a connecting node N10 ofa resistor R1 and a bipolar transistor BP1 and an input node N11 ofdifferential amplifiers 14 b, 14 c. The basic differential amplifier 34a compares a preset threshold voltage Vth10 (basic reference voltage)and a detection voltage N11 to output the comparison result as a basicdetection voltage N11. The differential amplifier 14 b compares athreshold voltage Vth11 and the basic detection voltage N11. Thedifferential amplifier 14 c compares the basic detection voltage N11 anda threshold voltage Vth12.

FIG. 11 shows the operation of the temperature detector 34 shown in FIG.10. The basic detection voltage N11 changes to high level when thedetection voltage N10>the threshold voltage Vth10 (FIG. 11( a)), and ina converse state, changes to low level (FIG. 11( b)). Here, thethreshold voltage Vth10 is set to a median value of the thresholdvoltages Vth1, Vth2.

The operations of the differential amplifiers 14 b, 14 c receiving thebasic detection voltage N11 and a flipflop 14 d are the same as those inthe first embodiment (FIG. 6). In this embodiment, the differentialamplifiers 14 b, 14 c receive the detection voltage N10 generated by atemperature detecting unit 14 a via a differential amplifier 34 a.Therefore, the detection voltage N10 that gently changes can beconverted to the basic detection voltage N11 that steeply changes. Thismakes it possible to make a steep voltage change at nodes N14, N12compared with that in the first embodiment.

The same effects as those of the above-described first embodiment arealso obtainable in this embodiment. Moreover, in this embodiment, sincethe voltage change of the nodes N14, N12 can be made steep, atemperature detection signal TDET can be surely generated even whenfluctuation in manufacturing conditions cause variances incharacteristics of the differential amplifiers 14 b, 14 c and thevariances cause an offset voltage. As a result, it is possible toprevent malfunction of the temperature detector 34 and to thereby surelygenerate the temperature detecting signal TDET even in a pseudo SRAMwhose specification is such that the threshold voltages Vth12, Vth11(boundary temperatures Tth1, Tth2) are close to each other.Alternatively, even when the threshold voltages Vth12, Vth11 vary due tofluctuation of manufacturing conditions, it is possible to surelygenerate the temperature detecting signal TDET.

FIG. 12 shows a fourth embodiment of the semiconductor integratedcircuit of the present invention. The same reference numerals andsymbols are used to designate the same elements as the elementsexplained in the first and second embodiments, and detailed explanationthereof will be omitted. The semiconductor integrated circuit of thisembodiment is formed on a silicon substrate as a pseudo SRAM, using aCMOS process.

The pseudo SRAM has a reference voltage generator 36, a temperaturedetector 38, a refresh timer 40, and a reference voltage setting circuit42 in place of the reference voltage generator 30, the temperaturedetector 14, the refresh timer 16, and the reference voltage settingcircuit 32 of the second embodiment. The other configuration issubstantially the same as that of the second embodiment.

The reference voltage generator 36 generates four threshold voltagesVth1, Vth2, Vth3, Vth4 corresponding to boundary temperatures Tth1,Tth2, Tth3, Tth4. The temperature detector 38 compares a voltagedetected according to the chip temperature of the pseudo SRAM with thethreshold voltages Vth1–4 to output 2-bit temperature detecting signalsTDET1–2 according to the comparison results. The refresh timer 40changes the timer cycle according to the temperature detecting signalsTDET1–2. This means that the generation interval (output frequency) of arefresh request signal RREQ is set according to the temperaturedetecting signals TDET1–2.

The reference voltage setting circuit 42 outputs 8-bit setting signalsSET in order to initially set respective values of the thresholdvoltages Vth1–4 to be generated by the reference voltage generator 36. 2bits of the setting signals SET are used to make the initial setting ofeach of the threshold voltages Vth1–4. The logic of the setting signalsSET is fixed during manufacturing processes of the pseudo SRAM as in thesecond embodiment.

FIG. 13 shows the temperature detector 38 shown in FIG. 12 in detail. Atemperature detecting unit 14 a, differential amplifiers 14 b, 14 c, anda flipflop 14 d for generating the temperature detecting signal TDET1are the same as those in the temperature detector 14 (FIG. 5) of thefirst embodiment. Further, the temperature detector 38 has differentialamplifiers 38 b, 38 c and a flipflop 38 d for generating the temperaturedetecting signal TDET2.

An output (a detection voltage N31) of the temperature detecting unit 14a is inputted in common to the differential amplifiers 14 b, 14 c, 38 b,38 c.

A generator for the temperature detecting signal TDET2 constituted ofthe differential amplifiers 38 b, 38 c, the flipflop 38 d, and so on isthe same as the generator for the temperature detecting signal TDET1constituted of the differential amplifiers 14 b, 14 c, the flipflop 14d, and so on. The differential amplifier 38 b compares the thresholdvoltage Vth3 and the detection voltage N31 to output the comparisonresult to an output node N36. The differential amplifier 38 c comparesthe detection voltage N31 and the threshold voltage Vth4 to output thecomparison result to an output node N38.

FIG. 14 shows the operation of the temperature detector 38 and therefresh timer 40 according to the change in the chip temperature. Thetemperature detector 38 sets the temperature detecting signal TDET1 tolow level indicating a high temperature state when detecting that thechip temperature of the pseudo SRAM shifts from a low temperature to ahigh temperature to exceed the boundary temperature Tth1 (represented bythe threshold voltage Vth1) (FIG. 14( a, b)). Therefore, the temperaturedetecting signal TDET1 changes from high level to low level only whenthe chip temperature exceeds the boundary temperature Tth2 and furtherexceeds Tth1. The temperature detector 38 sets the temperature detectingsignal TDET1 to high level indicating a low temperature state whendetecting that the chip temperature shifts from a high temperature to alow temperature to be lower than the boundary temperature Tth2(represented by the threshold voltage Vth2) (FIG. 14( c, d)). Therefore,the temperature detecting signal TDET1 changes from low level to highlevel only when the chip temperature becomes lower than the boundarytemperature Tth1 and further becomes lower than Tth2.

Further, the temperature detector 38 sets the temperature detectingsignal TDET2 to low level indicating a high temperature state whendetecting that the chip temperature of the pseudo SRAM shifts from a lowtemperature to a high temperature to exceed the boundary temperatureTth3 (FIG. 14( e, f)). Therefore, the temperature detecting signal TDET2changes from high level to low level only when the chip temperatureexceeds the boundary temperature Tth4 and further exceeds Tth3. Thetemperature detector 38 sets the temperature detecting signal TDET2 tohigh level indicating a low temperature state when detecting that thechip temperature shifts from a high temperature to a low temperature tobe lower than the boundary temperature Tth4 (represented by thethreshold voltage Vth4 (FIG. 14( g, h)). Therefore, the temperaturedetecting signal TDET2 changes from low level to high level only whenthe chip temperature becomes lower than the boundary temperature Tth3and further becomes lower than Tth4.

The temperature detector 38 maintains the level of the temperaturedetecting signals TDET1–2 when the chip temperature is between theboundary temperatures Tth1, Tth2 and between the boundary temperaturesTth3, Tth4. Further, when the chip temperature fluctuates around theboundary temperature Tth1, the boundary temperature Tth2, the boundarytemperature Tth3, and the boundary temperature Tth4, the level of thetemperature detecting signals TDET1–2 does not change. The boundarytemperatures are defined as Tth1>Tth2>Tth3>Tth4.

The refresh timer 40 sets the generation interval of a refresh requestsignal RREQ long when the logic values of the temperature detectingsignals TDET1–2 are “11”. The refresh timer 40 sets the generationinterval of the refresh request signal RREQ to a standard value when thelogic values of the temperature detecting signals TDET1–2 are “10”. Therefresh timer 40 sets the generation interval of the refresh requestsignal RREQ short when the logic values of the temperature detectingsignals TDET1–2 are “00”.

The same effects as those of the above-described first and secondembodiments are also obtainable in this embodiment. Moreover, in thisembodiment, the refresh frequency is delicately varied according to thechip temperature, which makes it possible to prevent unnecessaryoperation of the refresh timer 40 and so on and to further reduce thestandby current.

FIG. 15 shows a fifth embodiment of the semiconductor integrated circuitof the present invention. The same reference numerals and symbols areused to designate the same elements as the elements explained in thefirst embodiment, and detailed explanation thereof will be omitted. Thesemiconductor integrated circuit of this embodiment is formed on asilicon substrate as a DRAM having a self-refresh function, using a CMOSprocess. The DRAM executes a read operation, a write operation, or arefresh operation (auto-refresh) in response to an external command CMDduring a normal operation mode.

The DRAM executes the refresh operation during a self-refresh mode inresponse to a refresh request signal RREQ that is internally generatedperiodically. The DRAM is used as a work memory mounted on, for example,a notebook personal computer.

The DRAM has a command input circuit 44, a reference voltage generator46, a temperature detector 48, a refresh timer 50, and an operationcontrol circuit 52 in place of the command input circuit 10, thereference voltage generator 12, the temperature detector 14, the refreshtimer 16, and the operation control circuit 24 of the first embodiment.The other configuration is substantially the same as that of the firstembodiment.

During the normal operation mode, the command input circuit 44 (acommand decoder) receives the command signal CMD (for example, a rowaddress strobe signal /RAS, a column address strobe signal /CAS, a writeenable signal /WE, or the like) supplied via a command terminal. Thecommand input circuit 44 decodes the received command signal CMD (a readcommand, a write command, or an auto-refresh command) to output aninternal command signal ICMD for having a memory core 28 execute a readoperation, a write operation, or a refresh operation (auto-refresh).

Further, when receiving a self-refresh command via the command terminalCMD, the command input circuit 44 outputs a self-refresh signal SREF asthe internal command signal ICMD for shifting a chip from the normaloperation mode to the self-refresh mode. The command input circuit 44does not accept access requests (the read command, the write command)and the auto-refresh command during the self-refresh mode.

The reference voltage generator 46, the temperature detector 48, and therefresh timer 50 are activated for operation while receiving theself-refresh signal SREF. In other words, these circuits 46, 48, 50 stopoperating during the normal operation mode. The circuits unnecessaryduring the normal operation mode stop operating, which enables reductionin power consumption of the DRAM. The basic functions of the referencevoltage generator 46, the temperature detector 48, and the refresh timer50 are the same as those of the reference voltage generator 12, thetemperature detector 14, and the refresh timer 16 of the firstembodiment.

When receiving the read command, the write command, or the auto-refreshcommand from the command input circuit 44 during the normal operationmode, the operation control circuit 52 outputs a timing signal TIMINGfor having a memory core 28 execute the read operation, the writeoperation, or the refresh operation. When receiving the refresh requestsignal RREQ during the self-refresh mode, the operation control circuit52 outputs the timing signal TIMING for having the memory core 28execute the refresh operation. The operation of the operation controlcircuit 52 is the same as that of the operation control circuit 24 ofthe first embodiment. In this embodiment, however, a read request or awrite request does not conflict with a refresh request. Therefore, theoperation control circuit 52 does not have an arbiter.

The same effects as those of the above-described first embodiment arealso obtainable in this embodiment. Moreover, in this embodiment, thestandby current (self-refresh current) can be reduced also in the DRAMhaving the self-refresh mode.

FIG. 16 shows a sixth embodiment of the semiconductor integrated circuitof the present invention. The same reference numerals and symbols areused to designate the same elements as those explained in the first andsecond embodiments, and detailed explanation thereof will be omitted.The semiconductor integrated circuit of this embodiment is formed on asilicon substrate as a pseudo SRAM, using a CMOS process.

The pseudo SRAM has a reference voltage generator 54 and a referencevoltage setting circuit 56 in place of the reference voltage generator30 and the reference voltage setting circuit 32 of the secondembodiment. The other configuration is the same as that of the secondembodiment.

The reference voltage setting circuit 56 outputs three-bit fuse signalsFS in order to make initial setting of respective values of thresholdvoltages Vth1, Vth2 to be generated by the reference voltage generator54. The logic of the fuse signals FS is fixed in a manufacturing processof the pseudo SRAM. The reference voltage generator 54 generates thethreshold voltages Vth1, Vth2 according to the logic of the fuse signalFS, as in the second embodiment.

FIG. 17 shows in detail the reference voltage setting circuit 56 and thereference voltage generator 54 shown in FIG. 16. The reference voltagesetting circuit 56 has ROM units 56 a (ROM circuits) that output thefuse signals FS (FS1–3), respectively, whose logics are fixed in themanufacturing process of the pseudo SRAM. The configuration of the ROMunits 56 a is the same as that of the ROM units 32 c, 32 d of the secondembodiment (FIG. 9).

The reference voltage generator 54 has a decoder 54 a, variableresistors 54 b, 54 c (variable resistor unit), and a resistor array 54 dconstituted of three resistors connected in series. The decoder 54 adecodes the logic of the three-bit fuse signals FS1–3, activates one ofeight-bit decoded signals, and deactivates the other decoded signals.The eight-bit decoded signals are supplied commonly to both the variableresistors 54 b, 54 c.

The variable resistor 54 b (first variable resistor), the resistor array54 d, and the variable resistor 54 c (second variable resistor) areconnected in series between an internal power supply line VII and aground line VSS. The threshold voltage Vth1 (or Vth2) is generated froma connecting node of two adjacent resistors of the resistor array 54 d.

Resistance values of the variable resistors 54 b, 54 c are set accordingto the activated decoded signal. Here, the resistance values of thevariable resistors 54 b, 54 c are set such that the sum thereof is to bea constant value irrespective of the activated decoded signal.

That is, if the resistance value of the variable resistor 54 b increasesby a certain value, the resistance value of the variable resistor 54 cdecreases by the same value. Therefore, the sum of the resistance valuesof the variable resistor 54 b, the resistor array 54 d, and the variableresistor 54 c is unchanged. This makes it possible to keep constant avalue of current flowing through these resistors, so that a differencebetween the threshold voltages Vth1, Vth2 can be constant. The constantvoltage difference makes always constant a temperature differencebetween boundary temperatures Tth1, Tth2 at which a temperature detector14 changes the level of a temperature detecting signal TDET, whichfacilitates designing of the temperature detector 14.

Note that the threshold voltages Vth1, Vth2 drop by increasing theresistance value of the variable resistor 54 b and decreasing theresistance value of the variable resistor 54 c. Similarly, the thresholdvoltages Vth1, Vth2 rise by decreasing the resistance value of thevariable resistor 54 b and increasing the resistance value of thevariable resistor 54 c.

FIG. 18 shows an example of the reference voltage generator 54 shown inFIG. 17. The variable resistor 54 b has resistors R1 to R8 connected inparallel between the internal power supply line VII and the resistorarray 54 d, and switches SW connected in series to the respectiveresistors R1 to R8. Each of the switches SW receives one of the decodedsignals from the decoder 54 a. The switch SW receiving the activateddecoded signal turns on and the other switches SW turn off.Consequently, one of the resistors R1 to R8 is connected to the resistorarray 54 d. The resistors R1 to R8 increase in resistance value in orderof the resistor number. The resistors R1 to R8 are designed so thatdifferences in the resistance values between adjacent resistors (forexample, R1 and R2, or RS and R6) are all equal.

The structure of the variable resistor 54 c is the same as that of thevariable resistor 54 b except that the switches SW receive the decodedsignals in a reverse order. In this embodiment, the resistors R1, R8 ofthe variable resistors 54 b, 54 c are connected to the resistor array 54d, or the resistors R4, R5 of the variable resistors 54 b, 54 c areconnected to the resistor array 54 d, according to the decoded signals.Therefore, the sum of the resistance values of the variable resistors 54b, 54 c is always constant as described above.

The same effects as those in the above-described first and secondembodiments are also obtainable in this embodiment. Moreover, in thisembodiment, since the variable resistors 54 b, 54 c are used to form thereference voltage generator 54, different threshold voltages Vth1 (orVth2) can be generated from the same connecting node of the resistorarray 54 d. Therefore, the resistor array 54 d need not be provided witha switch (for example, the switch circuit 30 a in FIG. 9) for selectingthe threshold voltage Vth1 (or Vth2), so that the resistor array 54 dcan be simplified in its structure.

The sum of the resistance values of the variable resistors 54 b, 54 c ismade constant, which allows the threshold voltages Vth1, Vth2 to beincreased or decreased with a constant voltage difference therebetween.Consequently, the temperature detector 14 can change the level of thetemperature detecting signal TDET whenever a certain degree oftemperature change occurs. This can facilitate designing of circuitsinvolved in the temperature detection.

FIG. 19 shows an essential part of a seventh embodiment of thesemiconductor integrated circuit of the present invention. The samereference numerals and symbols are used to designate the same elementsas the elements explained in the first, fourth, and sixth embodiments,and detailed explanation thereof will be omitted. In this embodiment, areference voltage setting circuit 56 and a reference voltage generator58 are different from the reference voltage setting circuit 42 and thereference voltage generator 36 of the fourth embodiment. The otherconfiguration is the same as that of the fourth embodiment. Therefore,FIG. 19 shows only the reference voltage setting circuit 56 and thereference voltage generator 58 out of circuits constituting the pseudoSRAM. Since the reference voltage setting circuit 56 is the same as inthe sixth embodiment, detailed explanation thereof will be omitted.

The reference voltage generator 58 has a resistor array 54 e in place ofthe resistor array 54 d of the sixth embodiment. The other configurationis the same as the logical configuration of the reference voltagegenerator 54 of the six embodiment. The resistor array 54 e has fourresistors connected in series. A threshold voltage Vth1 (or Vth2, Vth3,Vth4) is generated from a connecting node of two adjacent resistors.

The same effects as those of the above-described first, second, fourth,and sixth embodiments are also obtainable in this embodiment.

FIG. 20 shows an eighth embodiment of the semiconductor integratedcircuit of the present invention. The same reference numerals andsymbols are used to designate the same elements as the elementsexplained in the first and fifth embodiments, and detailed explanationthereof will be omitted. The semiconductor integrated circuit of thisembodiment is formed on a silicon substrate as a DRAM having aself-refresh function, using a CMOS process. The DRAM executes a readoperation, a write operation, or a refresh operation (auto-refresh) inresponse to an external command CMD during a normal operation mode.

The DRAM executes the refresh operation during a self-refresh mode inresponse to a refresh request signal RREQ that is internally generatedperiodically. The DRAM is used as a work memory mounted on, for example,a notebook personal computer. The DRAM has a refresh timer 60 in placeof the refresh timer 50 of the fifth embodiment. The other configurationis the same as that of the fifth embodiment.

A reference voltage generator 46, a temperature detector 48, and therefresh timer 60 are activated for operation while receiving aself-refresh signal SREF. In other words, these circuits 46, 48, 60 donot operate in the normal operation mode.

Further, the refresh timer 60 initially sets a generation cycle of therefresh request signal RREQ short irrespective of a chip temperatureimmediately after the DRAM shifts from the normal operation mode to theself-refresh mode. In other words, even when the chip temperature islower than Tth2, the generation cycle of the refresh request signal RREQdoes not get longer. The other operations are the same as those of thefifth embodiment. Immediately after the shift to the self-refresh mode,a power supply voltage inside the DRAM is sometimes in an unstablestate, and so is an oscillation cycle of the refresh timer 60. Thegeneration cycle of the refresh request signal RREQ is set to a shortercycle until it is out of the unstable state, which can stabilizeoperation of the DRAM. In other words, it is possible to prevent loss ofdata retained in memory cells MC, resulting in improved reliability ofthe data retained in the memory cells MC.

FIG. 21 shows the operations of the temperature detector 48 and therefresh timer 60 in accordance with the change in the chip temperaturein the eighth embodiment. The refresh timer 60 sets the generation cycleof the refresh request signal RREQ short irrespective of the chiptemperature when the level of the self-refresh signal SREF changes fromlow level to high level and the DRAM shifts to the self-refresh mode(FIG. 21( a)). The refresh timer 60 changes the generation cycle of therefresh request signals RREQ depending on the chip temperature aftergenerating the refresh request signal RREQ twice. (FIG. 21( b)).

In this example, at the time of the shift from the normal operation modeto the self-refresh mode, the chip temperature is lower than Tth2.Therefore, the generation cycle of the refresh request signal RREQ is tobe set long originally. However, shortening the generation cycle of therefresh request signal RREQ for a period immediately after the shift tothe self-refresh mode until the internal operation of the DRAM isstabilized makes it possible to prevent loss of the data retained in thememory cells as described above.

The same effects as those in the above-described first and fifthembodiments are also obtainable in this embodiment. Moreover, in thisembodiment, the refresh timer 60 keeps the generation cycle of therefresh request signal RREQ short for a predetermined periodirrespective of the chip temperature after shifting to the self-refreshmode. Consequently, it is possible to surely refresh the memory cells MCduring a period in which the internal operation is unstable immediatelyafter the shift to the self-refresh mode, resulting in improvedreliability of the data retained in the memory cells MC.

FIG. 22 shows a ninth embodiment of the semiconductor integrated circuitof the present invention. The same reference numerals and symbols areused to designate the same elements as the elements explained in thefirst embodiment, and detailed explanation thereof will be omitted. Thesemiconductor integrated circuit of this embodiment is formed on asilicon substrate as a pseudo SRAM, using a CMOS process. The pseudoSRAM has a refresh timer 62 in place of the refresh timer 16 of thefirst embodiment. The other configuration is the same as that of thefirst embodiment.

The refresh timer 62 does not immediately change a generation cycle of arefresh request signal RREQ from a short cycle to a long cycle when thelevel of a temperature detecting signal TDET changes to high level dueto the shift of the chip temperature from low to high. The generationcycle of the refresh request signal RREQ is changed to a long cycleafter the refresh request signal is generated twice after thetemperature detecting signal TDET changes. Data reliability is higher ata short refresh cycle than at a long refresh cycle. Delaying the shifttiming when the shifting makes the data reliability lower makes itpossible to prevent loss of data retained in memory cells MC even if atemperature detector 14 erroneously detects a temperature droprepeatedly due to power supply noises and the like. As a result, it ispossible to improve reliability of the data retained in the memory cellsMC and to surely prevent malfunction of the temperature detector and ofthe semiconductor integrated circuit.

The refresh timer 62 changes the generation cycle of the refresh requestsignal RREQ from long to short immediately when the level of thetemperature detecting signal TDET changes to low level due to the shiftof the chip temperature from low to high. Quickly switching thegeneration cycle when the shifting makes the data reliability highermakes it possible to prevent loss of the data retained in the memorycells MC as described above.

FIG. 23 shows the operations of the temperature detector 14 and therefresh timer 62 in accordance with the change in the chip temperaturein the ninth embodiment. The level of the temperature detecting signalTDET changes from low level to high level only when the chip temperaturegoes lower than a boundary temperature Tth1 and further goes lower thanTth2 (FIG. 23( a)). At this time, the refresh timer 62 does notimmediately change the generation cycle of the refresh request signalRREQ but keeps it short (FIG. 23( b)). The refresh timer 62 sets thegeneration cycle of the refresh request signal RREQ long in response tothe second generation of the refresh request signal RREQ after the levelof the temperature detecting signal TDET changes to high level (FIG. 23(c)).

Meanwhile, the level of the temperature detecting signal TDET changesfrom high to low only when the chip temperature exceeds the boundarytemperature Tth2 and further exceeds Tth1 (FIG. 23( d)). At this time,the refresh timer 62 sets the generation cycle of the refresh requestsignal RREQ short in synchronization with a falling edge of thetemperature detecting signal TDET (FIG. 23( e)).

The same effects as those in the above-described first embodiment arealso obtainable in this embodiment. Moreover, in this embodiment, thegeneration cycle of the refresh request signal RREQ is switched with atime lag since the level change in the temperature detecting signal TDETfrom low (high temperature) to high (low temperature). This makes itpossible to surely prevent malfunction of the temperature detector 14and of the pseudo SRAM even when the detection results of thetemperature detector 14 greatly fluctuates due to power supply noisesand the like. As a result, reliability of the data retained in thememory cells MC can be improved.

Note that the above-described embodiments have described examples wherethe present invention is applied to the pseudo SRAM chip and the DRAMchip. The present invention is, however, not to be limited to suchembodiments. For example, the present invention may be applied to apseudo SRAM core and a DRAM core mounted on a system LSI.

The above-described embodiments have described examples where thepresent invention is applied to the pseudo SRAM or the DRAM. The presentinvention is not to be limited to such embodiments. For example, thepresent invention may be applied to a logic LSI and the like in whichthe cycle of an internal clock signal is changed according to the chiptemperature.

Further, the above-described second to fourth, sixth, seventh, and ninthembodiments may be applied to a DRAM instead of a pseudo SRAM.

The above-described second to fourth, sixth, and seventh embodimentshave described examples where the ROM circuit having the fuses is usedto make initial setting of the threshold voltages Vth1, Vth2, and othersin a manufacturing process (for example, a test process) of the pseudoSRAM. However, the present invention is not limited to such embodiments.For example, signal lines of the fuse signals FS1 and so on may bedirectly connected to the power supply line VII or the ground line VSSby a bonding wire in a manufacturing process (for example, an assemblyprocess) of the pseudo SRAM.

The foregoing second embodiment has described an example where thereference voltage setting circuit 32 and the reference voltage generator30 are formed for generation of each of the threshold voltages Vth1,Vth2. However, the present invention is not limited to such anembodiment. For example, the reference voltage setting circuit 32 may beformed in common to the threshold voltages Vth1, Vth2 and the settingsignals SET outputted from the reference voltage setting circuit 32 maybe outputted in common to the reference voltage generators 30corresponding to the respective threshold voltages Vth1, Vth2.

The foregoing sixth embodiment has described an example where theresistors R1 to R8 are connected in parallel between the power supplyline VII and the resistor array 54 d and between the ground line VSS andthe resistor array 54 d to form the variable resistors 54 b, 54 c.However, the present invention is not limited to such an embodiment. Forexample, as shown in FIG. 24, a plurality of resistors R9 may beconnected in series between the power supply line VII and the resistorarray 54 d and between the ground line VSS and the resistor array 54 dto form variable resistors 54 e, 54 f. In this case, resistance valuesof the resistors R9 are all set to the same value. The nine resistors R9are always connected between the power supply line VII and the groundline VSS, independent of an ON switch. The resistors R9 are manufacturedby utilizing diffusion resistance or wiring resistance generally.Therefore, even when the manufacturing conditions fluctuate in thesemiconductor manufacturing process, amounts of change in all theresistance values of the resistors R9 are the same. This enables all themanufactured resistors R9 to have the same resistance value. As aresult, it is possible to keep constant the sum of the resistance valuesof the variable resistors 54 e, 54 f even when the manufacturingconditions fluctuate.

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and scope ofthe invention. Any improvement may be made in part or all of thecomponents.

1. A semiconductor integrated circuit comprising: a temperature detectorsetting a level of a temperature detecting signal to a level indicatinga high temperature state when detecting that a chip temperature shiftsfrom low to high and is higher than a first boundary temperature,setting the level of the temperature detecting signal to a levelindicating a low temperature state when detecting that the chiptemperature shifts from high to low and is lower than a second boundarytemperature that is lower compared to the first boundary temperature,and maintains the level of the temperature detecting signal while thechip temperature is between the first boundary temperature and thesecond boundary temperature; and a control circuit changing its ownoperating state according to the level of the temperature detectingsignal, wherein said temperature detector includes: a temperaturedetecting unit generating a detection voltage corresponding to the chiptemperature; a first differential amplifier comparing a first referencevoltage corresponding to the first boundary temperature with thedetection voltage; a second differential amplifier comparing a secondreference voltage corresponding to the second boundary temperature withthe detection voltage; and a flipflop generating the level of thetemperature detecting signal according to results of the comparisonsfrom said first and second differential amplifiers.
 2. The semiconductorintegrated circuit according to claim 1 wherein said; temperaturedetecting unit has a resistor and a bipolar transistor, and generatessaid detection voltage corresponding to the chip temperature from aconnecting node of the resistor and the bipolar transistor that areconnected in series between a power supply line and a ground line. 3.The semiconductor integrated circuit according to claim 1, furthercomprising: a resistor array constituted of a plurality of resistors andgenerating the first and second reference voltages from connecting nodesof predetermined resistors, respectively, the plurality of resistorsarranged in series between the power supply line and the ground line;and a variable resistor unit connected in series to said resistor arrayand whose resistance value is changeable.
 4. The semiconductorintegrated circuit according to claim 3, wherein said variable resistorunit includes a first variable resistor disposed between said resistorarray and the power supply line and a second variable resistor disposedbetween said resistor array and the ground line.
 5. The semiconductorintegrated circuit according to claim 4, wherein resistance values ofthe first and second variable resistors are set such that a sum of theresistance values is to be always constant.
 6. The semiconductorintegrated circuit according to claim 3, further comprising a ROMcircuit presetting the resistance value of said variable resistor unit.7. The semiconductor integrated circuit according to claim 1, whereinsaid temperature detector further comprises a basic differentialamplifier comparing a basic reference voltage with the detection voltageto output a result of the comparison as a basic detection voltagewherein said first and second differential amplifiers receive said basicdetection voltage as said detection voltage.
 8. The semiconductorintegrated circuit according to claim 7, further comprising: a resistorarray constituted of a plurality of resistors and generating the firstand second reference voltages from connecting nodes of predeterminedresistors, respectively, the plurality of resistors arranged in seriesbetween the power supply line and the ground line; and a variableresistor unit connected in series to said resistor array and whoseresistance value is changeable.
 9. The semiconductor integrated circuitaccording to claim 8, wherein said variable resistor unit includes afirst variable resistor disposed between said resistor array and thepower supply line and a second variable resistor disposed between saidresistor array and the ground line.
 10. The semiconductor integratedcircuit according to claim 9, wherein resistance values of the first andsecond variable resistors are set such that a sum of the resistancevalues is to be always constant.
 11. The semiconductor integratedcircuit according to claim 8, further comprising a ROM circuitpresetting the resistance value of said variable resistor unit.
 12. Thesemiconductor integrated circuit according to claim 1, furthercomprising: a voltage generator generating a plurality of kinds ofvoltages; a switch circuit selecting two kinds from the plurality ofkinds of voltages to output the selected two as first and secondreference voltages; and a ROM circuit presetting voltages to be selectedby said switch circuit.
 13. The semiconductor integrated circuitaccording to claim 1, further comprising a memory array having dynamicmemory cells, wherein said control circuit is a refresh timer whichchanges a generation cycle of a refresh request signal according to thelevel of the temperature detecting signal, the refresh request signalbeing for refreshing the memory cells.
 14. The semiconductor integratedcircuit according to claim 13, further comprising: a command decoderdecoding a read command signal and a write command signal that areaccess requests supplied via an external terminal; and an operationcontrol circuit outputting a timing signal for putting said memory arrayinto operation in order to execute an access operation in response tothe read command signal and the write command signal and a refreshoperation in response to the refresh request signal, wherein saidoperation control circuit has an arbiter determining which one of theaccess operation and the refresh operation is to be given priority whenthe read command signal or the write command signal conflicts with therefresh request signal.
 15. The semiconductor integrated circuitaccording to claim 13, further comprising: a command decoder decoding aread command signal, a write command signal, and a self refresh commandsignal during a normal operation mode, the read and write commandsignals being access requests supplied via an external terminal, theself refresh command signal being for changing the normal operation modeto a self refresh mode; and an operation control circuit outputting atiming signal for putting said memory array into operation in order toexecute an access operation in response to the read command signal andthe write command signal and a refresh operation in response to therefresh request signal, wherein said refresh timer starts operating whensaid command decoder decodes the self refresh command signal.
 16. Thesemiconductor integrated circuit according to claim 15, wherein aftershifting to the self-refresh mode from the normal operation mode, saidrefresh timer fixes the generation cycle of the refresh request signalto a short cycle irrespective of the level of the temperature detectingsignal until it generates a predetermined number of refresh requestsignals, and after completion of the generation, said refresh timerchanges the generation cycle according to the level of the temperaturedetecting signal.
 17. The semiconductor integrated circuit according toclaim 13, wherein after a level change in the temperature detectingsignal, said refresh timer changes the generation cycle of the refreshrequest signal after it generates the predetermined number of refreshrequest signals.
 18. The semiconductor integrated circuit according toclaim 13, wherein after a level change in the temperature detectingsignal due to the shift of the chip temperature from high to low, saidrefresh timer lengthens the generation cycle after it generates thepredetermined number of refresh request signals according to the levelchange, and after a level change in the temperature detecting signal dueto the shift of the chip temperature from low to high, said refreshtimer shortens the generation cycle in response to the level change.